The present invention is generally directed to a method and apparatus for dark signal correction, and more particularly to a method and apparatus for correcting an unwanted dark signal of a photoelectric conversion device employed in spectrographic analysis of gaseous substances.
Instrumentation that analyzes gaseous substances is required in a variety of important applications. For example, during a surgical operation, patients are anesthetized through the careful administration of gases. The supply of these anesthetics must be regulated with great precision. In addition, the gases expelled in the patient's breath need to be monitored continuously to determine the condition of the patient. Instrumentation that analyzes gases exhaled by patients provide vital information to surgical personnel.
A particular class of instrumentation employs Raman spectroscopy to detect the presence and concentration of gaseous substances. Scattering of light by the Raman effect has received much attention from scientists since its original exposition by C. V. Raman in 1928. Instrumentation that employs the Raman effect provides a light, such as a laser beam, which illuminates molecules of a gas disposed within a sampling cell. Molecular vibrations of the gas cause the light to scatter off the illuminated gas molecules to produce scattered light in a process which shifts the frequency of the scattered light by exactly the vibrational frequency of the molecule. The frequency shift of the spectral signal is characteristic of the gas being analyzed and is independent of the frequency of the illuminating light. Thus, the Raman scattered light can be used to infer properties of the gas being analyzed, such as chemical composition and concentration.
The Raman scattered light is collected from the gas disposed within the sampling cell and constituent frequency components of the light are analyzed and detected. The analyzed frequency components are used to produce a spectrogram, which can be displayed on a display device. By interpreting the spectrogram, the presence and concentration of different types of constituent gas molecules in the gas can be deduced. Though the Raman effect can be employed to provide a powerful analytical tool, it should be noted that in general, intensity of the light produced by the Raman effect is extremely weak. The Raman scattered light has an extremely low intensity relative to intensity of radiation used to stimulate the effect.
Instrumentation that employs the Raman effect is described in U.S. Pat. No. 4,648,714 entitled "Molecular Gas Analysis By Raman Scattering in lntracavity Laser Configuration" by Benner et al. issued Mar. 10, 1987, and in U.S. Pat. No. 4,784,486 entitled "Multichannel Molecular Gas Analysis By Laser-Activated Raman Light Scattering" by Van Wagenen et al. Because each of these patents provides helpful background information, they are incorporated herein by reference. The instrumentation described in each of these two patents provides a laser beam that illuminates molecules of a gas disposed within a sampling cell. Raman scattered light generated by the Raman-effect is collected from the gas disposed within the sampling cell.
Each patent includes discussions focussing on a respective means for detecting constituent frequency components of the Raman scattered light. Van Wagenon et al. teach that a photomultiplier tube is used to detect the Raman scattered light. Benner et al. teach that any suitable state of the art detector such as a photo diode, intensified diode array or photomultiplier tube may be used.
Though photoelectric conversion devices such as photo diodes, photomultipliers or the like can be employed as discussed by Van Wagehen and Benner to detect Raman scattered light, certain difficulties still remain. A practical limitation of such photoelectric conversion devices is that if the devices are exposed to Raman scattered light, then the devices produce electrical signals that include a desired signal intermixed with an unwanted dark current signal. The desired signal is generated in response to the Raman scattered light. The unwanted dark current signal is generated independently of any illumination of the photoelectric conversion device. If the photoelectric conversion device is shielded from exposure to light, a dark signal alone is directly measurable at an output of the device. Thus it is easy to analyze the dark signal alone without producing the desired signal. Unfortunately, there is no way known for the photoelectric conversion device to directly produce the desired signal without also producing the dark signal intermixed therewith. It is difficult to isolate the desired signal because the desired signal is inherently intermixed with the unwanted dark signal.
Problems arise because the dark signal tends to obscure the desired signal. Such a negative effect of the dark signal is especially prominent in Raman spectroscopy wherein the photoelectric conversion device is employed to detect the Raman scattered light. Because the intensity of light produced by the Raman effect is extremely weak, the desired signal produced by the photoelectric conversion device in response to the Raman scattered light is a weak signal. Because the desired signal is so weak, the desired signal is especially susceptible to being obscured by the dark signal intermixed therewith.
Some correction schemes provide a partial solution to the problems caused by the dark current signal. A fixed correction scheme employs a fixed compensating signal in an attempt to correct the electrical signal produced by the photoelectric conversion device exposed to Raman scatted light. The fixed compensating signal is subtracted from the electric signal to produce a corrected signal. The fixed compensation scheme has some effectiveness at removing fixed signal artifacts of the dark signal contributed by charge injection of multiplexers, reset circuits and op-amps commonly used in conjunction with photodiodes and diode arrays. However, taken as a whole, the fixed correction scheme has only limited success because the dark signal also includes variable signal artifacts. For example, experiments show that the dark signal is a strong function of temperature of the photoelectric conversion device. Therefore, it is theorized that the dark signal includes a variable thermal artifact. For most semiconductor photoelectric conversion devices, the dark signal approximately doubles for every 7 degrees Celsius increase in temperature of the photoelectric conversion device. Because the fixed compensating signal does not adapt to variable signal artifacts of the dark signal, a better correction scheme is needed.
A variable correction scheme employs a pair of similarly constructed photoelectric conversion devices that are thermally coupled so as to be in thermal equilibrium with one another. A first member of the pair of conversion devices is exposed to Raman scattered light in a similar manner as discussed previously, thereby producing a first electric signal that includes a desired signal intermixed with a first dark signal. A second dark signal is generated by shielding a second member of the pair of photoelectric conversion devices from exposure to light. Because the first and second devices are similarly constructed, it is theorized that the first dark signal is similar to the second dark signal. Furthermore, since the pair of devices are in thermal equilibrium with one another, it is theorized that thermal variations in the first dark signal are tracked by the second dark signal. The variable compensation scheme is intended to compensate for the effect of the first dark signal upon the desired signal by subtracting the second dark signal from the first electric signal to produce a corrected signal that is thermally adjusted.
Unfortunately, the variable correction scheme adds noise to the corrected signal produced thereby. Each dark signal produced includes a respective noise artifact. For example, a first noise artifact is present in the first dark signal, and a second noise artifact is present in the second dark signal. Therefore, in addition to inheriting the first noise artifact from the first dark signal, the corrected signal produced by the variable correction scheme also inherits the second noise artifact from the second dark signal. Accordingly, the corrected signal produced by the variable correction scheme includes approximately twice the noise of the corrected signal produced by the fixed correction scheme discussed previously. A further problem with the variable correction scheme is that it is inherently inefficient since a pair of conversion devices are required while only one member of the pair directly contributes to generating the desired signal.
While the fixed correction scheme and the variable correction scheme each make contributions, they also each have limitations. Because the desired signal produced in Raman spectroscopy is extremely weak, any corrected signal derived therefrom is especially susceptible to being obscured by additional noise. Therefore an efficient dark signal correction method and apparatus are needed to compensate for fixed and variable signal artifacts of the dark signal, while reducing noise added by the correction method and apparatus. Furthermore, the dark signal correction method and apparatus should remain efficient while regularly monitoring and correcting the intermixed signal, so that a corrected signal produced thereby can be advantageously used to provide a corrected real-time display to a doctor monitoring a patient undergoing anesthesia.